Lane Reduction (Road Diet)

Some roads have more travel lanes than necessary, and the width of the excess lanes could be freed up for other uses. Space may be better used for bicycle lanes, parking, or wider pedestrian buffers or sidewalks (with curb realignment). A traffic analysis should be conducted to determine whether a lane reduction is feasible, (i.e., vehicle capacity exceeds existing and projected volumes; many roadways were built without such analysis and in other cases traffic volumes have receded over time). The most common road diet configuration involves converting a four-lane road to three lanes, with one travel lane in each direction and a center two-way left-turn lane (TWLTL), often supplemented with painted, textured, or raised center islands. Left-turning drivers can exit the traffic stream and wait in the TWLTL, while through traffic can maintain a fairly constant speed. Four-to-three-lane conversions should be considered for roadways with documented safety concerns, moderate volumes (less than 15,000 Average Daily Traffic, up to 25,000 ADT in special cases), and along priority bicycle and walking routes.

There are many opportunities to perform road diets, particularly on roadways with wider cross sections, one-way streets (which may have excess capacity), and although not as common, where volumes are low on a three-lane road (one lane in each direction with a TWLTL) can be converted to two.

Extra roadway space can be reallocated for other roadway users to improve safety, comfort, and convenience (see list above). Reconstruction projects may allow for curb lines to be moved to narrow the roadway. With the additional space created from restriping or reconstruction, space can be redistributed for the following uses:

Purpose

Reducing lane numbers remedies excess capacity situations. More space for bicyclists, pedestrians, and parking is provided. Reducing the apparent width of the road provides median refuge and improves safety for all street users. It improves social interaction and enhances livability of the street.

Lane reductions (i.e., road diets) optimize street space to benefit all users. Lane reductions help improve safety and comfort for pedestrian as well as bicyclists. Reducing the number of lanes on a multilane roadway can help improve sight distances for left-turning vehicles and create space for bicycle, transit, and/or parking lanes.

Considerations

There are a number of factors to consider when determining the appropriateness of a lane reduction. These include:

The volumes and types of traffic, left-turn movements, multimodal crash data, roadway widths, sight distance, and the number of driveways.

Determine if and how alternative routes will be impacted by a lane reduction.

Lane reductions on higher volume roadways (more than 20,000 ADT) should be studied carefully to ensure that traffic controls and access are appropriate for larger volumes of traffic.

Estimate the level of service (LOS) for all roadway users, and understand that roadway treatments that result in a lower LOS for vehicles can increase LOS for transit riders, bicyclists, and pedestrians, especially those with disabilities, the elderly, and young children. Other factors may include the importance a particular street plays in the pedestrian or bicycle network, and the relationship between creating more livable streets and supporting economic development.

Consider designs that incorporate raised medians and left-turn bays to help eliminate the potential for TWLTL to be used as acceleration lanes by some motorists.

On constrained corridors that are major bicycle or transit routes and where vehicle capacity warrants the existing lane configuration, consider removing on-street parking to provide dedicated bicycle and/or transit facilities. Parking removal should be considered on a case-by-case basis using engineering judgment and community input.

In its guidance memo on road diets, the FHWA recommends consideration of improvements to turn lanes, signing, pavement markings, traffic control devices, transit stops, and bicycle and pedestrian facilities. FHWA also notes the classic four-to-three-lane road diet is very compatible with single-lane roundabouts.

Consider documenting before-and-after studies of the conversion for safety and traffic flow improvements.

Estimated Cost

The cost for restriping a mile of four-lane street to one lane in each direction plus a TWLTL and bike lanes is about $5,000 to $20,000 per mile, depending on the amount of lane lines that need to be repainted. The estimated cost of extending sidewalks or building a raised median is much higher and can cost $100,000 per mile or more. If a reconfiguration is done after repaving or with an overlay, and curbs do not need to be changed, there is little or no cost for space reallocations accomplished through new striping.

Safety Effects

A summary of studies that have looked at the safety effects of lane reductions can be found here.